Heat-shrinkable barrier packaging films have long been used for the packaging of a variety of products.
In particular, barrier heat shrinkable films comprising PVDC barrier layers are known and especially appreciated for their very good barrier properties—both under dry and under humid conditions—and for the excellent moisture barrier performance.
However incorporation of PVDC in multilayer films is not devoid of processability problems, due to its chemical and thermal instability, which result in formation of hydrochloric acid, bubbles and in browning of the tape.
In particular, in order to coextrude films comprising both PVDC and high melting polymers—such as polyolefins or polyamides with a melting point generally lower than 200-225° C.—insulated extrusion dies together with incapsulation techniques have been developed to prevent PVDC thermal degradation. Furthermore, in order to be successful in PVDC extrusion, a precise control of the extrusion process is required, in particular on residence time, equipment design and materials used.
However, coextrusion of PVDC together with much higher melting polymers, such as aromatic polyesters having melting temperatures well above, still represents a technical challenge.
Through the years, several efforts have been done in the technical field to increase the stiffness of PVDC comprising films in order to improve their abuse resistance and machinability.
An improved stiffness of the film is generally desirable as results in packages with less leakers, leakers which are due to accidental openings or ruptures during the packaging process or handling of the packages. More rigid films also provide for an improved machinability, which allows to decrease the rejects and increase the speed of the packaging cycles. In fact, a film having good machinability is less subject to creasing, folding, seal pleats, edge curls, or jamming and can be more easily used with any packaging machine. Additionally, more stiff films provide for flexible containers which are easier to be aligned and loaded with the product. Finally, they generally show improved stability during bubble orientation and better printability.
However, in case of shrinkable films, it has been observed that, an increase in the stiffness—achievable for instance by increasing the polyamide content or the number of polyamide layers in the film—often results in inferior shrinking properties or, in case a high shrink is anyway achieved, in worsened optical characteristics after shrink (e.g., gloss, haze). Furthermore, the resulting film is less planar which renders the subsequent converting operations much more difficult.
Concerning the shrinking properties, an ideal packaging film should have the correct balance of free shrink, maximum shrink tension and residual shrink tension in order to provide packages with an appealing appearance and a satisfactory functionality, appearance and functionality which should be preserved under the most common packaging and storage conditions and over time for the entire package life.
Any deviation of the optimal values of said shrinking properties may be detrimental to the performance of the film in the final package.
For instance, too low free shrink values may result in pack appearance unacceptable for the customer due to the looseness of the film and to the presence of wrinkles.
This is particularly true for the vacuum packaging of meat products, particularly fresh meat products. Upon evacuating the atmosphere from the package followed by heat-sealing of the film, the resulting closed package should tightly shrink around the meat product. A film endowed with a sufficiently high free shrink retracts against the product, reducing the excess of film protruding away from the packaged product and improving the appearance and the function of the package.
At this shrinking step, it is essential that the films develop proper free shrink values in both the directions together with an appropriate shrink force. This force must be high enough to tightly enclose the product within the film but without crashing or excessively distorting the final package.
The free shrink and the maximum shrink tension, i.e. the maximum value of tension developed by the films during the heating/shrinking process, are thus parameters very important for achieving an optimal package appearance.
Another important requirement is that the packages should remain tight overtime, during handling and storage.
One common inconvenient that occurs during the storage into the refrigerator is the so called “package relaxation”, namely the loss of pack tightness and the appearance of anti-aesthetical wrinkles and pleats in the packaging film. Package relaxation is not only undesirable for purely aesthetical reasons—the presence of wrinkles in the film of the package is not attractive per se—but also because it may impair the visual inspection of the packaged product, and thus instill doubts concerning the freshness and the proper storage of the food.
As far as the Applicant knows, there are several patents which mention the possibility to associate aromatic polyester layers with a PVDC barrier layer in a multilayer shrinkable film.
However, these documents either do not really exemplify structures comprising both these materials or, if they do, they describe manufacturing processes clearly unsuitable or do not provide a disclosure sufficient for replicating said structures, as process and equipment details, needed to allow their real manufacture without damaging the PVDC layer, are totally missing.
EP2147783A1 discloses shrinkable films with improved stack-sealability comprising a PVDC internal barrier layer and an outer polyester layer, in which the polyester is a high melting polyester. The only example discloses a film comprising polybutylene terephthalate (with melting point of 225° C.), characterized by a free shrink at 90° C. of 32% in MD and 32% in TD. No other shrink property is reported therein.
The description is silent on specific equipment and conditions needed for coextrusion, subsequent orientation of the tape and final cooling of the films. Concerning coextrusion, it only mentions that: “The preferred method for the production of the film is coextrusion in a multilayer die” (par. 0049). Concerning orientation and cooling conditions, it refers in very broad terms to “a double bubble method described in U.S. Pat. No. 3,456,044” (par. 0051) but does not provide any detail in merit.
EP2030784 in the name of Cryovac Inc. discloses films comprising an outer heat-sealable layer, an internal PVDC barrier layer and an external abuse layer. No one of the films really exemplified therein includes an aromatic polyester layer, but lower melting polyamides or polystyrenes. This document does not provide any specific shrink value for the exemplified films nor any teaching for specific maximum shrink tension and residual shrink tension values.
In the manufacture process, the PVDC barrier layer is placed in the coating and extruded together with the outer layer. The exemplified films do not experience PVDC thermal damages, as the polymers used in the outer layers, namely the polyamides, their admixtures or the polystyrenes, have melting point significantly lower than aromatic polyesters.
U.S. Pat. No. 8,039,070 in the name of Curwood Inc. relates to peelable barrier shrinkable films. The description generally mentions polyesters among several other polymers as possible components of the film outer layer, but none of the films exemplified therein comprises PVDC and high melting polymers, especially high melting aromatic polyesters. Barrier materials may be selected among ethylene vinyl alcohol copolymers (EVOH), polyacrylonitriles, polyamides, vinylidene chloride copolymers (PVDC), polyglycolide copolymers, and mixtures thereof. All the examples relate to EVOH barrier films.
WO2005011978 in the name of Cryovac Inc. discloses biaxially oriented heat shrinkable films comprising a heat sealable layer, an outer polyester layer and an internal EVOH barrier layer. The manufacturing process comprises coextrusion of all the layers (no substrate plus coating). The EVOH layer is resistant to thermal stress and it is not damaged by the high temperatures applied to extrude the polyesters. The document does not consider PVDC as alternate material for the barrier layer.
U.S. Pat. No. 6,699,549 in the name of Kureha Chemical Inc. discloses films comprising one heat-sealable layer, one outer polyester layer, and optionally an EVOH barrier layer. PVDC as barrier material is not considered. In the manufacturing process, all the layers are coextruded together (no substrate plus coating) as preserving the barrier layer from thermal damaging is not an issue here.
U.S. Pat. No. 6,764,729 in the name of Cryovac Inc. discloses heat shrinkable films comprising a first outer heat-sealable layer and an outer layer comprising polyolefin, polystyrene or a second polyamide. None of the films exemplified therein shows a polyester layer or a PVDC barrier layer. They are manufactured according to a process comprising coextrusion i.e all layers extruded together (no substrate plus coating).
U.S. Pat. No. 4,064,296 in the name of Grace W R & Co. discloses heat shrinkable high barrier, cross-linked films. The barrier polymer is HEVA, the outer layer is made of polyolefins. Further materials are generally listed as possible components of additional layers, but none of the films exemplified therein comprises a polyester layer and/or a PVDC barrier layer.